Persistent stimulation of striatal dopamine receptors increases the calmodulin content in the supernatant fraction. This increase appears to be triggered by membrane phosphorylation catalyzed by a cAMP-dependent protein-kinase. In the supernatant fraction prepared from striatal slices cAMP-phosphodiesterase has a biphasic kinetic profile. Incubation of striatal slices with dopamine or opiate receptor agonists changes the biphasic profile to a monophasic function. Haloperidol prevents the changes elicited by both types of agonists. In addition, the changes in PDE kinetic properties and calmodulin content elicited by morphine were also curtailed by naltroxone and striatal deafferentation.